Corner coordinates issue

[ldjohn at u...]

Hey Frank,

As the LAS support team moves forward with the incorporation of GDAL into
our image I/O layer, issues arise from time to time. The issue of the day
is corner coordinates.

The LAS package supports multiple geographic image formats. Among those
are NDF and Landsat 7 HDF. These two formats pose a problem with respect
to GDAL because they contain multiresolution data with corner coordinates
*centered on the corner pixels*. This means that the geographic
area covered varies across bands. (Yeah, we don't like it either,
but we have to support it.)

The ASCII art below attempts to describe the situation. In this figure,
band 2 is at a higher resolution than band 1. Both bands share the same
corner coordinates, which are located at the centers of the corner pixels
for each resolution. Thus band 2 actually covers a smaller geographic
region than band 1.

+-------+-------+-------+-------+-------+
band 1 -->| +---+-+-------+-------+-------+-+---+ |
| | + | | | | | | + | |
| +---+ | | | | +---+ |
+-+---+-+-------+-------+-------+-----+-+
| +---+ | |
band 2 ---->| | | |
| +---+ | |
| | | | |
| +---+ + = corner coordinate | |
| | | | |
| +---+ | |
| | | | |
| +---+ | |
| | | | |
| +---+ | |
+-+---+-+ +-----+-+
| +---+ | | +---+ |
| | + | | | | + | |
| +---+-+-----------------------+-+---+ |
+-------+-----------------------+-------+

Have you considered options for covering such scenarios with
center-of-pixel corner coordinates? If we have to handle it ourselves,
we'll probably use various metadata strings to carry the additional,
unsupported corner coordinate info along, but that would most likely
break the GDAL ability to automatically handle format conversions for us.

This doesn't appear to be an easy problem to resolve, but if you have any
ideas for best handling these formats in GDAL, we would be happy to hear
them.

Thanks.


Lowell
-- 
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Lowell D. Johnson ldjohn at u...
Software Engineering
Raytheon Systems
EROS Data Center
47914 252nd Street, Mundt Federal Bldg
Sioux Falls, SD 57198
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